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Microscopic evaluation of the absolute fluence distribution of a large-area uniform ion beam using the track-etching technique
Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Microscopic evaluation of the absolute fluence distribution of a large-area uniform ion beam using the track-etching technique Akane Kitamura (Ogawa) a,⇑, Tetsuya Yamaki b, Yosuke Yuri a, Shin-ichi Sawada b, Takahiro Yuyama a a Department of Advanced Radiation Technology, Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan b High Performance Polymer Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2012 Received in revised form 7 May 2013 Accepted 13 May 2013 Available online 20 June 2013 Keywords: Track etching Heavy-ion beam Fluence distribution Uniform beam
a b s t r a c t The absolute fluence distribution of a large-area uniform beam was investigated microscopically via track etching of an Ar-irradiated polyethylene terephthalate (PET) film. The irradiated sample was divided equally into 64 pieces, for each of which the track-pore densities were counted over a 12 17 lm2 microscopic area near the center. For comparison, the relative intensity distribution was obtained by measuring the optical density of a similarly irradiated Gafchromic film at a resolution of 500 500 lm2 and then taking the measured value at the center of each of the 64 areas. The relative standard deviations of the distributions were in good agreement despite the difference in the observed resolution area and the small sample number. It was, therefore, confirmed that track etching is a reliable technique for evaluating absolute fluence and that a uniform intensity distribution of the beam was microscopically realized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The transverse intensity distribution of an ion beam is crucial in various radiation experiments because it directly influences the irradiation effect. A uniform-beam formation/irradiation system using multipole magnets has been developed for material and biological studies at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of Japan Atomic Energy Agency (JAEA) [1]. In the system, the relative transverse intensity distribution of the beam has been measured using Gafchromic radiochromic films (Ashland Inc.) [2]. The film was originally produced for dose evaluation of X-ray and gamma-ray radiotherapy, but it can also be applied to the measurement of the intensity distribution of ion beams. The measurement technique is based on the fact that the optical density of the film increases linearly with the particle fluence [3]. The intensity distribution obtained with a Gafchromic film agrees well with the relative transverse intensity distribution obtained from a real-time ion-induced fluorescence image [4]. However, in practice, it is difficult to precisely obtain the absolute fluence distribution from the optical density, because the coloration of the film is subject to various external conditions, such as the elapsed time between irradiation and film reading and the environmental temperature and humidity.
⇑ Corresponding author. Tel.: +81 27 346 9350; fax: +81 27 346 9690. E-mail address: [email protected] (A. Kitamura (Ogawa)). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.027
In this study, the absolute particle fluence distribution was evaluated on the microscopic level using the track-etching technique in order to overcome the above problems with the distribution measurement using Gafchromic films. The technique involves the irradiation of a material (e.g., polymer films) with energetic heavy ions followed by preferential chemical etching of the ion tracks [5]. The areal distribution of the etched-pore density is then compared to the relative intensity distribution measured using the Gafchromic films. The absolute fluence distribution at the microscopic level is often necessary for some research studies using ion-track technology [6]. 2. Experimental Polyethylene terephthalate (PET) films (25-lm-thick, Hoechst JAPAN) and Gafchromic films (104-lm-thick, HD-810, Ashland Inc.) were irradiated in air with a 520-MeV 40Ar uniform beam in the azimuthally varying field (AVF) cyclotron at TIARA [7]. The kinetic energy on the suface of the sample was estimated to be approximately 330 MeV using the Monte Carlo simulation Stopping and Range of Ions in Matter [8]. The irradiated area was approximately 50 50 mm2 and the average fluence was 1.0 108 ions/cm2, which was estimated from the beam current measured near the target using a Faraday cup, the irradiation time, and the coloration of the Gafchromic film. A perturbed, non-uniform beam with a different distribution was also employed for comparison.
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A. Kitamura (Ogawa) et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
50 mm
HD-810 film or PET film
1 secon
64 secons HD-810 film
PET film
SEM observaon area
12 μm
50 mm
500 μm
Area from which the opcal density was extracted
500 μm
track pores
17 μm
Fig. 1. Scheme for measurement of the track-pore density on a PET film and extraction of the optical densities from an HD-810 film.
For counting incident ions precisely, all ion tracks need to be developed individually into etched pores. The etching conditions were selected as follows. The irradiated PET films were exposed to ultraviolet light in air for 1 h in order to increase the sensitivity of the ion tracks [9,10] and then etched in 1 M NaOH (solvent: 10 wt% ethanol in water) at 80 °C for 10 min without stirring [11]. Next, they were washed with a large amount of water and dried at 40 °C for 4 h in a vacuum. Fig. 1 shows the scheme for estimating the areal distribution of the etched-pore density on the PET film. The samples were equally divided into 64 sections of approximately 6 mm square. The number of track-etched pores in the central region (12 17 lm2) of each section was counted with a scanning electron microscope (SEM) (TM3000, Hitachi High-Technologies Corp.) after the surface was coated with gold using a plasma coater (JFC-1600, JEOL). The diameter of the pores was typically a few hundred nm, which was small enough such that the effect of track overlapping could be ignored. Accordingly, the density was exactly the same as the absolute fluence. The irradiated Gafchromic films were read using a flat-bed scanner (Cannon LiDE50) in order to digitize them into TIFF images with 16-bit RGB color intensity values at a resolution of 500 500 lm2. The sensitivity of the film coloration is the highest for the red color component [3], and thus the optical density d was determined from the red color value R of the image using the equation d = log10 (216 1/R). In the present fluence range, the d value was proportional to the Ar fluence. A total of 64 square-grid optical densities were extracted at intervals of approximately 6 mm from the original measured data so that the intensity distribution could be directly compared to that of the PET film (Fig. 1). For both the track-etched PET and Gafchromic films, the uniformity of the intensity distribution was evaluated by taking the relative standard deviations (RSD) of the pore density and the d value over the 64 sections.
1 cm
(a)
3. Results and discussion The relative intensity distributions for the Ar uniform beam obtained from the evaluation of the HD-810 film is shown in Fig. 2. Fig. 2(a) shows the color image at a 500 lm resolution, while Fig. 2(b) shows an image reconstructed using the 64 sections extracted from Fig. 2(a). The RSD of the reconstructed image was estimated to be 12%, which was equal to that in Fig. 2(a), based on composed of 104 points. This result indicated that the use of 64 data points was sufficient for the estimation of the beam unifor-
(b) Fig. 2. (a) Two-dimensional (2D) distribution of the relative intensity obtained for the HD-810 film irradiated using the uniform beam. (b) 2D distribution of the 64 square-grid points extracted at intervals of approximately 6 mm from Fig. 1.
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A. Kitamura (Ogawa) et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
mity. Note that although the RSD of 12% obtained here is small enough for the present study, a beam with a higher uniformity can actually be achieved with the uniform-beam formation/irradiation system, as reported in Ref. [1]. Fig. 3 shows the absolute fluence distribution obtained from the microscopic track-pore densities of the PET film irradiated with the same uniform Ar beam. The density was exactly the same as the absolute fluence, because the track-pores formed on the PET film were the evidence of the incident ions. Thus, the present technique for evaluating the fluence distribution using a PET film is accurate. It was also found that the average track-pore density for all the 64 sections was 0.9 108 ions/cm2, and the RSD of the distribution was 12%, which was in good agreement with the RSD obtained for the HD-810 film (Fig. 2) despite the large difference in the observation resolution for the two films. From this result, it was revealed that for the uniform beam formed in the system used in this study, the uniformity observed in a microscopic region of 102 lm2 was the same as that in a relatively macroscopic region of 105 lm2. The number distribution based on the 64 sections is shown in Fig. 4 as a function of the fluence on both films. On the HD-810
film, the fluence distribution was approximately estimated based on the relationship between the average fluence and the average optical density. One large peak can be seen on each of the curves, which were fitted to the experimental data by the Gaussian distribution. While the widths of these peaks are nearly the same because the same RSD was obtained for each film (12%), the peak positions are different; the track-etched PET film exhibits a peak maximum at 0.9 108 ions/cm2, while the peak on the HD-810 film reaches a maximum at 1.0 108 ions/cm2. The difference seems to be reasonable considering the accuracy of the beam current measurement. The intensity distribution was also investigated using an intentionally perturbed non-uniform beam for verification with a different distribution. Figs. 5 and 6 show the relative fluence distribution on the HD-810 film and the absolute fluence distribution on the track-etched PET film, respectively. The average of the absolute fluence was estimated to be 0.9 108 ions/cm2. The maximum and minimum were 0.5 108 and 1.6 108 ions/cm2, respectively. The RSD on the PET film deteriorated to 23%, which was again very close to that on the HD-810 film (22%).
1.8
1.4 1.2 1.0
Track-pore density [ 108 ions/cm2]
1.6
0.8 0.6 Fig. 3. 2D distribution of the track-pore density obtained from SEM images of the PET film. The irradiation conditions were the same as those in Fig. 2.
Fig. 5. 2D distribution of the relative intensity obtained from the HD-810 film irradiated with the perturbed non-uniform beam.
1.8
1.4 1.2 1.0
Track-pore density [ 108 ions/cm2]
1.6
0.8 0.6 Fig. 4. Distribution of the number of sections as a function of the fluence on the HD810 film shown in Fig. 2(b) and the PET film shown in Fig. 3. The solid lines denote the result of the Gaussian curve fit to the experimental data points.
Fig. 6. 2D distribution of the track-pore density obtained from the PET film irradiated with the perturbed non-uniform beam. The irradiation conditions were the same as those in Fig. 5.
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On the basis of these observations, the present microscopic technique for evaluating the beam intensity distribution using the track pores was statistically validated and confirmed to be highly reliable. 4. Conclusion The absolute particle fluence distribution of a large-area uniform ion beam was evaluated on the microscopic level using track etching of a PET film. The present technique for evaluating the fluence using the track pores is reliable because it directly reflects an actual incident ion. The microscopic absolute fluence was determined from the track-pore density, which was found to be slightly lower than the estimated value obtained using a Gafchromic film. However, the RSDs of the distributions in the PET and Gafchromic films were in good agreement, despite the difference in the observed resolution area and the small sample number. Thus, the uniform intensity distribution was microscopically revealed in our uniform-beam formation system using the track-etching technique.
References [1] Y. Yuri, T. Ishizaka, T. Yuyama, I. Ishibori, S. Okumura, K. Yoshida, Nucl. Instr. Meth. A 642 (2011) 10–17. [2]. [4] T. Yuyama, Y. Yuri, T. Ishizaka, I. Ishibori, S. Okumura, Proceedings of the 1st International Beam Instrumentation Conference, Tsukuba, Japan (2012), (in press). [5] R. Spohr, in: K. Bethge (Ed.), Ion Tracks and Microtechnology, Vieweg Verlag, Braunschweig, Germany, 1990. [6] T. Yamaki, J. Power Sour. 195 (2010) 5848–5855. [7] K. Arakawa, Y. Nakamura, W. Yokota, M. Fukuda, T. Nara, T. Agematsu, S. Okumura, I. Ishibori, T. Karasawa, R. Tanaka, A. Shimizu, T. Tachikawa, Y. Hayashi, K. Ishii, T. Satoh, In Proceedings of the 13th International Conference on Cyclotrons and their Applications, Vancouver, Canada, 1992. 119. [8] J.F. Ziegler, Nucl, Instrum. Meth. Phys. Res. Sec. B 219–220 (2004) 1027. [9] V.I. Kuznetsov, A.Yu. Didyk, P.Yu. Apel, Nucl. Tracks Radiat. Meas. 19 (1991) 919–924. [10] P. Yu Apel, Radiat. Meas. 25 (1995) 667–674. [11] A. Hiroki, M. Asano, T. Yamaki, M. Yoshida, Chem. Phys. Lett. 406 (2005) 188– 191.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Microscopic evaluation of the absolute fluence distribution of a large-area uniform ion beam using the track-etching technique Akane Kitamura (Ogawa) a,⇑, Tetsuya Yamaki b, Yosuke Yuri a, Shin-ichi Sawada b, Takahiro Yuyama a a Department of Advanced Radiation Technology, Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan b High Performance Polymer Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan
a r t i c l e
i n f o
Article history: Received 30 November 2012 Received in revised form 7 May 2013 Accepted 13 May 2013 Available online 20 June 2013 Keywords: Track etching Heavy-ion beam Fluence distribution Uniform beam
a b s t r a c t The absolute fluence distribution of a large-area uniform beam was investigated microscopically via track etching of an Ar-irradiated polyethylene terephthalate (PET) film. The irradiated sample was divided equally into 64 pieces, for each of which the track-pore densities were counted over a 12 17 lm2 microscopic area near the center. For comparison, the relative intensity distribution was obtained by measuring the optical density of a similarly irradiated Gafchromic film at a resolution of 500 500 lm2 and then taking the measured value at the center of each of the 64 areas. The relative standard deviations of the distributions were in good agreement despite the difference in the observed resolution area and the small sample number. It was, therefore, confirmed that track etching is a reliable technique for evaluating absolute fluence and that a uniform intensity distribution of the beam was microscopically realized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The transverse intensity distribution of an ion beam is crucial in various radiation experiments because it directly influences the irradiation effect. A uniform-beam formation/irradiation system using multipole magnets has been developed for material and biological studies at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of Japan Atomic Energy Agency (JAEA) [1]. In the system, the relative transverse intensity distribution of the beam has been measured using Gafchromic radiochromic films (Ashland Inc.) [2]. The film was originally produced for dose evaluation of X-ray and gamma-ray radiotherapy, but it can also be applied to the measurement of the intensity distribution of ion beams. The measurement technique is based on the fact that the optical density of the film increases linearly with the particle fluence [3]. The intensity distribution obtained with a Gafchromic film agrees well with the relative transverse intensity distribution obtained from a real-time ion-induced fluorescence image [4]. However, in practice, it is difficult to precisely obtain the absolute fluence distribution from the optical density, because the coloration of the film is subject to various external conditions, such as the elapsed time between irradiation and film reading and the environmental temperature and humidity.
⇑ Corresponding author. Tel.: +81 27 346 9350; fax: +81 27 346 9690. E-mail address: [email protected] (A. Kitamura (Ogawa)). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.027
In this study, the absolute particle fluence distribution was evaluated on the microscopic level using the track-etching technique in order to overcome the above problems with the distribution measurement using Gafchromic films. The technique involves the irradiation of a material (e.g., polymer films) with energetic heavy ions followed by preferential chemical etching of the ion tracks [5]. The areal distribution of the etched-pore density is then compared to the relative intensity distribution measured using the Gafchromic films. The absolute fluence distribution at the microscopic level is often necessary for some research studies using ion-track technology [6]. 2. Experimental Polyethylene terephthalate (PET) films (25-lm-thick, Hoechst JAPAN) and Gafchromic films (104-lm-thick, HD-810, Ashland Inc.) were irradiated in air with a 520-MeV 40Ar uniform beam in the azimuthally varying field (AVF) cyclotron at TIARA [7]. The kinetic energy on the suface of the sample was estimated to be approximately 330 MeV using the Monte Carlo simulation Stopping and Range of Ions in Matter [8]. The irradiated area was approximately 50 50 mm2 and the average fluence was 1.0 108 ions/cm2, which was estimated from the beam current measured near the target using a Faraday cup, the irradiation time, and the coloration of the Gafchromic film. A perturbed, non-uniform beam with a different distribution was also employed for comparison.
48
A. Kitamura (Ogawa) et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
50 mm
HD-810 film or PET film
1 secon
64 secons HD-810 film
PET film
SEM observaon area
12 μm
50 mm
500 μm
Area from which the opcal density was extracted
500 μm
track pores
17 μm
Fig. 1. Scheme for measurement of the track-pore density on a PET film and extraction of the optical densities from an HD-810 film.
For counting incident ions precisely, all ion tracks need to be developed individually into etched pores. The etching conditions were selected as follows. The irradiated PET films were exposed to ultraviolet light in air for 1 h in order to increase the sensitivity of the ion tracks [9,10] and then etched in 1 M NaOH (solvent: 10 wt% ethanol in water) at 80 °C for 10 min without stirring [11]. Next, they were washed with a large amount of water and dried at 40 °C for 4 h in a vacuum. Fig. 1 shows the scheme for estimating the areal distribution of the etched-pore density on the PET film. The samples were equally divided into 64 sections of approximately 6 mm square. The number of track-etched pores in the central region (12 17 lm2) of each section was counted with a scanning electron microscope (SEM) (TM3000, Hitachi High-Technologies Corp.) after the surface was coated with gold using a plasma coater (JFC-1600, JEOL). The diameter of the pores was typically a few hundred nm, which was small enough such that the effect of track overlapping could be ignored. Accordingly, the density was exactly the same as the absolute fluence. The irradiated Gafchromic films were read using a flat-bed scanner (Cannon LiDE50) in order to digitize them into TIFF images with 16-bit RGB color intensity values at a resolution of 500 500 lm2. The sensitivity of the film coloration is the highest for the red color component [3], and thus the optical density d was determined from the red color value R of the image using the equation d = log10 (216 1/R). In the present fluence range, the d value was proportional to the Ar fluence. A total of 64 square-grid optical densities were extracted at intervals of approximately 6 mm from the original measured data so that the intensity distribution could be directly compared to that of the PET film (Fig. 1). For both the track-etched PET and Gafchromic films, the uniformity of the intensity distribution was evaluated by taking the relative standard deviations (RSD) of the pore density and the d value over the 64 sections.
1 cm
(a)
3. Results and discussion The relative intensity distributions for the Ar uniform beam obtained from the evaluation of the HD-810 film is shown in Fig. 2. Fig. 2(a) shows the color image at a 500 lm resolution, while Fig. 2(b) shows an image reconstructed using the 64 sections extracted from Fig. 2(a). The RSD of the reconstructed image was estimated to be 12%, which was equal to that in Fig. 2(a), based on composed of 104 points. This result indicated that the use of 64 data points was sufficient for the estimation of the beam unifor-
(b) Fig. 2. (a) Two-dimensional (2D) distribution of the relative intensity obtained for the HD-810 film irradiated using the uniform beam. (b) 2D distribution of the 64 square-grid points extracted at intervals of approximately 6 mm from Fig. 1.
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A. Kitamura (Ogawa) et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
mity. Note that although the RSD of 12% obtained here is small enough for the present study, a beam with a higher uniformity can actually be achieved with the uniform-beam formation/irradiation system, as reported in Ref. [1]. Fig. 3 shows the absolute fluence distribution obtained from the microscopic track-pore densities of the PET film irradiated with the same uniform Ar beam. The density was exactly the same as the absolute fluence, because the track-pores formed on the PET film were the evidence of the incident ions. Thus, the present technique for evaluating the fluence distribution using a PET film is accurate. It was also found that the average track-pore density for all the 64 sections was 0.9 108 ions/cm2, and the RSD of the distribution was 12%, which was in good agreement with the RSD obtained for the HD-810 film (Fig. 2) despite the large difference in the observation resolution for the two films. From this result, it was revealed that for the uniform beam formed in the system used in this study, the uniformity observed in a microscopic region of 102 lm2 was the same as that in a relatively macroscopic region of 105 lm2. The number distribution based on the 64 sections is shown in Fig. 4 as a function of the fluence on both films. On the HD-810
film, the fluence distribution was approximately estimated based on the relationship between the average fluence and the average optical density. One large peak can be seen on each of the curves, which were fitted to the experimental data by the Gaussian distribution. While the widths of these peaks are nearly the same because the same RSD was obtained for each film (12%), the peak positions are different; the track-etched PET film exhibits a peak maximum at 0.9 108 ions/cm2, while the peak on the HD-810 film reaches a maximum at 1.0 108 ions/cm2. The difference seems to be reasonable considering the accuracy of the beam current measurement. The intensity distribution was also investigated using an intentionally perturbed non-uniform beam for verification with a different distribution. Figs. 5 and 6 show the relative fluence distribution on the HD-810 film and the absolute fluence distribution on the track-etched PET film, respectively. The average of the absolute fluence was estimated to be 0.9 108 ions/cm2. The maximum and minimum were 0.5 108 and 1.6 108 ions/cm2, respectively. The RSD on the PET film deteriorated to 23%, which was again very close to that on the HD-810 film (22%).
1.8
1.4 1.2 1.0
Track-pore density [ 108 ions/cm2]
1.6
0.8 0.6 Fig. 3. 2D distribution of the track-pore density obtained from SEM images of the PET film. The irradiation conditions were the same as those in Fig. 2.
Fig. 5. 2D distribution of the relative intensity obtained from the HD-810 film irradiated with the perturbed non-uniform beam.
1.8
1.4 1.2 1.0
Track-pore density [ 108 ions/cm2]
1.6
0.8 0.6 Fig. 4. Distribution of the number of sections as a function of the fluence on the HD810 film shown in Fig. 2(b) and the PET film shown in Fig. 3. The solid lines denote the result of the Gaussian curve fit to the experimental data points.
Fig. 6. 2D distribution of the track-pore density obtained from the PET film irradiated with the perturbed non-uniform beam. The irradiation conditions were the same as those in Fig. 5.
50
A. Kitamura (Ogawa) et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 47–50
On the basis of these observations, the present microscopic technique for evaluating the beam intensity distribution using the track pores was statistically validated and confirmed to be highly reliable. 4. Conclusion The absolute particle fluence distribution of a large-area uniform ion beam was evaluated on the microscopic level using track etching of a PET film. The present technique for evaluating the fluence using the track pores is reliable because it directly reflects an actual incident ion. The microscopic absolute fluence was determined from the track-pore density, which was found to be slightly lower than the estimated value obtained using a Gafchromic film. However, the RSDs of the distributions in the PET and Gafchromic films were in good agreement, despite the difference in the observed resolution area and the small sample number. Thus, the uniform intensity distribution was microscopically revealed in our uniform-beam formation system using the track-etching technique.
References [1] Y. Yuri, T. Ishizaka, T. Yuyama, I. Ishibori, S. Okumura, K. Yoshida, Nucl. Instr. Meth. A 642 (2011) 10–17. [2]